Cho Seung-Hui's rampage was the worst, but hardly the first. What have we learned about why such people kill?

If you want a sense of just how terrible Monday's crimes were, here's something to try: imagine yourself committing them. It's easy enough to contemplate what it would feel like to rob a bank or steal a car; you might even summon a hint of the outlaw frisson that could make such crimes seem appealing. But picture yourself as Cho Seung-Hui, the 23-year-old student responsible for the Virginia Tech bloodbath, walking the halls of the school, selecting lives to extinguish and then ... extinguishing them. It is perhaps a measure of our humanity that we could sooner imagine ourselves...